Many systems are known that comprise agents that are activated by, or respond to, light. There are generally two mechanisms of action. The agent may itself directly absorb incident light and in doing so bring about the desired response or activation. Alternatively, the agent does not directly absorb the incident light, but a light-absorbing additive is present to initiate the response or activation.
An example of the first mechanism is laser imaging with the colour change agent ammonium octamolybdate (AOM) using a CO2 laser that emits radiation in the mid-infrared region of the spectrum at approximately 10.6 microns. AOM is particularly useful for laser imaging using a CO2 laser, absorbing 10.6 micron radiation and undergoing a white to black colour change reaction. As a result AOM has negligible absorbance in the visible region of the spectrum (400 to 700 nm). Consequently ink formulations that comprise AOM appear white. As AOM changes into black, aesthetically pleasing printed substrates are obtained together with a high level of contrast which is required for machine readability such as with barcodes.
AOM can also be used to exemplify the second mechanism. AOM has negligible absorbance in the near infrared (NIR) region of the spectrum (780 to 2500 nm). However, there are applications where it would be desirable to employ a smaller NIR fibre laser (rather than a bulky CO2 laser). Therefore, to image AOM based ink formulations with a NIR fibre laser, operating for example with an emission wavelength of 1070 nm, a NIR absorbing component has to be included in the system. Examples of the sort of NIR absorbing compounds that have been used to do this are inorganic pigments such copper (II) hydroxyl phosphate (CHP), organic NIR dyes such as N,N,N′,N′-tetrakis(4-dibutylaminophenyl)-p-benzoquinone bis(iminium hexafluoroantimonate) (ADS-1065) and Lumogen IR-1050, and conductive polymers such as PEDOT (sold as Baytron P).
However, because NIR absorbers have absorbance maximum peaks in the NIR region there is usually a tailing of this peak into the visible region of the spectrum. Consequently, most NIR absorbers appear visibly coloured and thus substrates that comprise NIR absorbers also appear visibly coloured. The depth of colour depends on the concentration of NIR absorbing additive used. In the case of inks, substrates coated with formulations that comprise NIR absorbers can appear visibly coloured rather than white. One of the main advantages of AOM is the fact that it is colourless and substrates coated with AOM based inks for use in CO2 laser imaging appear white. However, substrates coated with inks comprising AOM and a NIR absorber, for use in NIR fibre laser imaging, appear coloured. The effect is particularly noticeable with white substrates such as plastic film and paper. This is not only unappealing to the eye but can hinder machine code readability.